1. Field of the Invention
The present invention relates to a recorder which turns on and off power from a power supply to record on a record medium by recording elements.
2. Description of the Prior Art
In a prior art recorder such as a thermal head driver of a thermal printer, a carrier (or a thermal head) is moved relative to a main body of the recorder. Therefore, flexible wires for controlling drive of heat generating elements must be connected between a power supply and a central processing unit (controller or CPU) on a stationary unit and the carrier.
The heat generating elements are driven by the power supply, the CPU, a shift register (temporary memory) and a driver which drives the heat generating elements in accordance with a pattern signal from the shift register and controlled by a constant voltage for regulating a supply voltage from the power supply at a constant level.
FIG. 1 shows a circuit configuration of a prior art thermal head driver.
In FIG. 1, a CPU 1, a shift register 2 and a driver 3 which is switching means to energize and deenergize heat generating elements 12 are located on a stationary unit. The shift register 2 is controlled by a print data signal 4, a print command signal 5 and a timing signal 6 from the CPU 1 and the driver 3 is controlled in accordance with a pattern signal 7 from the shift register. On the other hand, a power line 9 from a positive power supply 8 is connected to the heat generating elements 12 (only three of those are shown although 12 to 24 elements are usually provided) on a thermal head on a moving unit through a constant voltage circuit 10, and negative power lines 13 connected to the heat generating elements 12 are connected to a negative power supply 14.
The positive power line 9 and as many negative power lines as the number of heat generating elements 12 are accommodated in a flexible cable 15.
In operation, data signals 4 serially sent from the CPU 1 are sequentially loaded to the shift register 2 which controls the driver 3 in a serial-in, parallel-out fashion to print a desired pattern. In this manner, the data are printed by repeating the pattern printing.
In such a prior art thermal head driver, if the power lines 9 and 13 in the relatively long flexible cable 15 have resistances, voltage drops occur because of relatively high currents flowing therethrough and the voltage drops vary by the change in the number of dots (the number of heat generating elements) to be driven. As a result, the supply voltages to the heat generating elements 12 vary depending on the number of dots and the densities are not uniform. When the number of heat generating elements 12 is 24, a current in the order of 100mA, for example, flows per heat generating element and the power supply voltages when all of the 24 dots are printed and those when one dot is printed are significantly different if the power lines include the resistances. Thus, the print densities are not uniform.
In the prior art recorder, in order to keep the voltages constant independently from the number of dots, large diameter power lines 9 and 13 (or wide lines when a flexible printed circuit lines are used) are utilized to minimize the voltage drops.
However, when the large diameter power lines 9 and 13 are used, the diameter of the flexible cable 15 increases and it occupies more space. In addition, the flexible cable becomes less flexible reliability is lowered and manufacturing cost increases.